If the blood stain is quite recent, the colouring matter will be hæmoglobin only. This easily dissolves out in water, and when sufficiently diluted gives the spectrum of oxy-hæmoglobin, which on the addition of ammonia, together with a small quantity of the double tartrate, a small piece of ferrous salt, and stirring carefully without the admission of air, changes the spectrum of reduced hæmoglobin. When stirred again, so as to expose the solution as much as possible to air, the two bands reappear; on gradually adding citric acid in small quantities the colour begins to change, and the bands are seen to gradually fade away; if there should have been much blood present, a band appears in the red; the further addition of ammonia makes all clear again, but does not restore the original bands, because the hæmoglobin has been permanently changed into hæmatin. This reaction alone distinguishes blood from most other colouring matters, since other substances after being changed by acids are restored by alkalies to their original state. There are many other curious facts connected with the spectroscopic analysis of blood, which are fully explained and illustrated by Dr. Maemunn in his book on “The Use of the Spectroscope in Medicine,” and also in Dr. Thudicum’s[36] reports and charts, which are the most complete. Sir George Stokes, F.R.S., was one of the first to show the essential value of the spectral phenomena of hematine, and who proved, after Hoppe had first drawn attention to the fact, that this colouring matter is capable of existing in two states of oxidation, and that a very different spectrum is produced according as the substance, which he termed _cruorine_, is in a more or less oxidised condition. The chart appended to his paper[37] affords an imperfect representation of the changes seen in the spectrum. [Illustration: No. 1.--Arterial Blood, Scarlet Cruorine. No. 2.--Venous Blood, Purple Cruorine. No. 3.--Blood treated with Acetic Acid. No. 4.--Solution of Hæmatin. Fig. 199.--Sir George Stokes’ Chart of the Absorption Bands of Blood.] Proto-sulphate of iron, or proto-chloride of tin, causes the reduction of the colouring-matter, but, on exposure to air, oxygen is absorbed, and the solution again exhibits the spectrum characteristic of the more oxidised state. The different substances obtained from blood colouring-matter produce different bands. Thus, _hæmatin_ gives rise to a band in the red spectrum D; _hæmato-globulin_ produces two bands, the second twice the breadth of the first in the yellow portion of the spectrum between the lines D and E, No. 1. The absorption-bands differ according to the strength of the solution employed, and the medium in which the blood-salt is dissolved; but an exceedingly minute proportion dissolved in water is sufficient to bring out very distinct bands. B represents the red end of the spectrum and G the green as it approaches the violet end.